It is well known that peroxygen bleaching agents, e.g., perborates, percarbonates, perphosphates, persilicates etc., are highly useful for chemical bleaching of stains found on both colored and white fabrics. Such bleaching agents are most effective at high wash solution temperatures, i.e., above about 70.degree. C. In recent years, attempts have been made to provide bleaching compositions that are effective at lower wash solution temperatures, i.e., between room temperature and 70.degree. C. In consequence, bleaching agents have been investigated which exhibit their optimum bleach activity in this temperature range. These low temperature bleaches are useful in a variety of products intended for use under machine or hand-wash conditions, e.g., additive, pre-additive or soak-type laundry compositions as well as all-purpose detergent compositions.
A very effective class of low temperature bleach system comprises a peroxy bleach compound and an organic peracid precursor which react together to form the organic peracid in the wash solution. Examples of detergent compositions incorporating bleaching agents of this type are disclosed in U.S. Pat. No. 2,362,401 (Reicher et al), U.S. Pat. No. 3,639,248 (Moyer) and in British Pat. Nos. 836,988 and 855,735.
It is well-known, however, that bleach activator containing detergent compositions suffer a number of technical problems which until now have limited their commercial applicability and market success. The underlying problem is that of activator instability, i.e., the tendency of the activator to degrade by hydrolysis and perhydrolysis reactions under the alkaline and oxidizing conditions typically encountered in detergent compositions during storage. This leads not only to loss of bleaching efficacy but also to degradation of other sensitive ingredients in the detergent formula, for example, perfumes, optical brighteners, enzymes, dyes etc.
In the art, two major approaches have been used to tackle the instability problem. In the first approach, the activator is protected from its hostile alkaline/oxidizing environment by agglomeration, coating or encapsulation with a non-hygroscopic, preferably hydrophobic agglomerating, coating or encapsulating material (see for instance British Pat. Nos. 1,441,416 and 1,398,785, U.S. Pat. Nos. 3,494,786 (Neilson), 3,494,787 (Lund and Neilson) and 3,441,507 (Schiefer)). This technique suffers the disadvantage, however, that to be efficaceous, the agglomerating or coating material must be so water-impervious as to considerably inhibit the rate of release of bleach activator into the detergent wash liquor. This leads to diminished bleach effectiveness and increased cost. Where, on the other hand, a hydrophilic agglomerating or coating agent is used, for instance, a water-soluble liquid nonionic surfactant, the hygroscopicity of the product is such that no meaningful improvement in activator stability can be achieved. For this reason, a number of patents (for instance British Pat. No. 1,561,333 and European patent application No. 6655) advocate the use of normally solid nonionic surfactants as agglomerating or coating agents and this can indeed lead to some improvement in hygroscopicity. Once again, however, agglomerates of this type typically display poor activator-release characteristics and diminished bleach effectiveness.
In the second approach to improving activator stability, the activator is incorporated in the detergent composition in the form of relatively coarse-sized particles (see, for instance, U.S. Pat. No. 4,087,369), the object being to reduce interaction of the activator with its environment by minimizing the surface/unit weight of the activator. This approach suffers the disadvantage, however, that the rate of dispersion and solubilization of the activator is so slow as to considerably increase the risk of fabric damage known as "pinpoint spotting". In essence, "pinpoint spotting" is a local bleach effect caused by slow dissolution of individual particles of the bleach system resulting in a locally high concentration of the bleaching agent at the fabric surface. High solubilization rate is thus seen to be critical for avoiding problems of damage to fabrics, but in as much as high solubilization rate has traditionally implied either a high activator surface/unit weight or agglomeration with a hygroscopic agglomerating agent, it follows that the twin aims of improving fabric safety and activator stability have been to a large degree mutually exclusive.
The present invention seeks, as one of its objectives, to resolve these conflicting requirements by providing a matrix of materials in particulate form that has excellent granular physical characteristics, activator stability and rate of solution/dispersion characteristics; that delivers these benefits in a composition comprising high levels of detergent functional nonionic surfactants; and which also delivers these benefits in a detergent composition prepared from highly alkaline and oxidizing detergent components.
As used herein below, the terms "bleach activator" and "organic peroxy acid bleach precursor" are directly equivalent to one another.